1. Field of the Invention
The present invention relates to a ventilator of the type having an inspiratory unit and an expiratory valve for regulating a flow of breathing gas, and a control unit for controlling the inspiratory unit and the expiratory valve.
2. Description of the Prior Art
Mechanical ventilation is employed to control or support a patient's breathing. Breathing gas at a positive pressure is supplied to the patient. In many instances, ventilation is a vital necessity for the patient, however, the treatment in itself is not without risks. Numerous studies of animals suggest that ventilation can initiate or aggravate lung damage. A major contributory reason for this is the mechanical stress to which the pulmonary system may be subjected during ventilation. In particular, damage can develop or be aggravated when alveoli in the lungs cyclically open and collapse during inspiration and expiration.
Moreover, some patients suffer from such severe conditions that using a treatment that can cause some damage to the lungs is the only way to save the patient's life.
One way to prevent alveolar collapse is to impose a positive pressure on the lungs, even during expiration, with a positive end-expiratory pressure (PEEP). However, PEEP cannot be set too high, since a high PEEP can subject the lungs to harmfully high pressure during inspiration. An excessive inspiratory pressure can impede blood perfusion in the lungs (causing poorer oxygenation of the blood) and even inflicting damage to pulmonary tissue.
Recruitment phases can be employed instead of a constantly high PEEP. During a recruitment phase, the pulmonary alveoli are opened with a pressure (usually higher than the normal inspiratory pressure for the patient), enabling them to remain open when exposed to a lower pressure during a subsequent period of treatment. The recruitment phase is repeated as needed.
A number of procedures are known for instituting the recruitment phase. One is often referred to as the 40/40 method. This means that the lung is subjected to a pressure of about 40 cmH2O for up to 40 seconds.
This is a static procedure and rather hard on the lung and the patient, but it is a simple maneuver producing clear results in the form of improved oxygenation of the blood.
Another procedure involves the use of a number of brief pressure pulses, e.g. three, each lasting 15 seconds.
In principle, the effect is the same as in the aforementioned instance. However, static methods have a major shortcoming in the buildup of carbon dioxide in the lungs during the recruitment phase.
A third known procedure involves a successive increase in pressure in the lungs during, in principle, normal breaths until a satisfactory degree of opening is achieved.
This is without doubt the easiest method on the lung and patient, but it is highly demanding and difficult for staff to implement.